Phase transitions in the borate minerals from the Kłodawa Salt Dome (Central Poland) as indicators of temperature processes in salt diapirs

Abstract

Wachowiak, J., Tobo³a, T., 2014. Phase tran si tions in the bo rate min er als from the K³odawa salt dome (cen tral Po land) as indi ca tors of tem per a ture pro cesses in salt diapirs. Geo log i cal Quar terly, 58 (3): 543-554, doi: 10.7306/gq.1170The diapiric struc tures of the Pol ish Low lands are tec toni cally deeply seated down to the autochthonous Zechstein strata at a depth of ca. 6 km. In the pro cess of deep burial and halokinetic diapirism, the salt rocks were sub jected to diagenetic and meta mor phic trans for ma tions, with the tem per a ture be ing an es sen tial fac tor. Con sid er ing the ther mal gra di ent, a tem per ature of up to ca. 200°C can be achieved in the salt dome within a depth range from sev eral hun dred metres to 6 km, which may lead to trans for ma tions of the ma jor ity of salt min er als. Phase tran si tions of two bo rate min er als -boracite and congolite from the K³odawa salt dome -pro vide ev i dence for higher tem per a tures in the salt dome rocks. The authigenic euhedral crys tals of those bo rate min er als dis play their ex ter nal habitus in am bi ent tem per a ture in the form of reg u lar sym me try (F43c -pseudo-reg u lar poly he drons), whereas their in ter nal struc ture is lower: orthorhombic (Pca2 1 ) for boracite and rhombohedral (R3 C ) for congolite. The heat ing and cool ing of boracite and congolite crys tals show re vers ible phase tran si tion. At a tem per a ture of ca. 270°C, boracite crys tals change their sym me try: orthorhombic « cu bic. In the case of congolite three revers ible phase tran si tions within a tem per a ture range of 50-339°C can be ob served: rhombohedral « monoclinic « orthorhombic « cu bic sym me try. Those phase tran si tions, con firmed ex per i men tally in our study, clearly doc u ment at least lo cal oc cur rences of tem per a tures ex ceed ing 339°C in the K³odawa salt dome.

Inclusions in anhydrite crystals from blue halite veins in the Kłodawa Salt Dome (Zechstein, Poland)

“…More over, the mi gra tion of hy dro ther mal so lu tions through the K³odawa Salt Dome might have re sulted in the ap pear ance of min er als which are not directly re lated to these for ma tions, as boracite and congolite (Wachowiak and Pieczka, 2012;Wachowiak and Tobo³a, 2014). In flu ence of in creas ing tem per a ture in the diapir was also mentioned by Wag ner and Burliga (2014) for the Stink ing Shale and the Main Do lo mite.…”

“…The leach ing prop er ties of so lu tions com bined with high tem per a ture and pres sure might have in fluenced the tec tonic move ments be cause the so lu tions worked like a lu bri cant be tween the salt blocks. More over, their mi gra tion af fected chem i cally and ther mally the sur round ing salts caus ing, for ex am ple, pre cip i ta tion of min er als which are not ge net i cally re lated to the ma rine evaporites, e.g., bo rates (Wachowiak and Pieczka, 2012;Wachowiak and Tobo³a, 2014).…”

Evidence of high-temperature rock salt transformations in areas of occurrence of borate minerals (Zechstein, Kłodawa salt dome, Poland)

“…Their oc cur rence can not be ex plained by sed i men tation, diagenetic or epigenetic pro cesses re sult ing from the burial of the Zechstein salt-bear ing for ma tion, or by tec tonic activ ity dur ing de vel op ment of the salt domes. These phe nom ena in clude, for ex am ple, zones of bo rate min eral oc cur rences (boracite and congolite) in rock salt and in po tas sium-mag nesium salts (Wachowiak and Pieczka, 2012;Wachowiak and Tobo³a, 2014). The form of oc cur rence of those min er als as cubic blocks (congolite) or reg u lar poly he dra (boracite) in di cates that they crys tal lised in reg u lar sys tems and in high-tem per ature con di tions.…”

“…The form of oc cur rence of those min er als as cubic blocks (congolite) or reg u lar poly he dra (boracite) in di cates that they crys tal lised in reg u lar sys tems and in high-tem per ature con di tions. Lab o ra tory stud ies on phase tran si tions in ar ti ficially formed and nat u ral bo rate min er als clearly de ter mine the tem per a tures above which those min er als crys tal lise in a reg ular sys tem (for congolite: 339°C and for boracite: 270°C - Car pen ter, 1996, 1997;Wachowiak and Tobo³a, 2014). "Blue ha lite" veins rep re sent an other type of salt rock al tered at high tem per a tures (Tobo³a, 2016).…”

“…Pre vi ous study (Wachowiak and Pieczka, 2012;Wachowiak and Tobo³a, 2014) con cerned the boracite min er als from K³odawa salt dome and their phase trans for ma tion. The purpose of the pres ent study was to check whether fac tors in di cating high-tem per a ture rock trans for ma tion have been pre served in the salt-bear ing rocks con tain ing bo rate min er als.…”